The present invention relates to a liquid crystal optical element which controls the transmittance, scattering or reflectance of an element by the application conditions of an electric field and which is useful for e.g. a display element, an optical shutter or a light control element, and a liquid crystal composition to be used therefor.
At present, a liquid display element such as TN, STN or TFT is practically used. As other liquid crystal display elements, practical use of e.g. a ferroelectric/antiferroelectric liquid crystal display element characterized in high speed response/memory function, and a guest/host display element attracting attention by colorization, is being studied.
Further, a liquid display element (hereinafter referred to also as cholesteric LCD) by means of a chiral nematic (cholesteric) liquid crystal, has a memory function and is capable of presenting a bright reflection type display, and thus attention is drawn thereto as a liquid crystal display element of next generation, and its practical use is being studied. The chiral nematic liquid crystal has a memory function such that once a predetermined voltage is applied, the alignment state can be maintained to be stable in a selectively reflective planar state or a scattering focalconic state, without applying a voltage thereafter
The chiral nematic liquid crystal can be obtained by mixing a nematic liquid crystal and an optically active compound. Such a chiral nematic liquid crystal is interposed between a pair of parallel substrates to constitute a liquid crystal optical element. When the center axis (the helical axis) of a helix of a helical structure wherein directors of the liquid crystal rotate every constant period, is aligned in a perpendicular direction on an average to the substrates, the liquid crystal optical element having the chiral nematic liquid crystal interposed, will reflect circularly polarized light corresponding to the direction of the helix. The center wavelength of the reflected light will be a product of the distance (the helical pitch) on the helical axis where directors of the liquid crystal parallel with the substrate surfaces undergo one rotation by the helix, and the average refractive index at the two-dimensional surface parallel to the substrate surfaces of the nematic liquid crystal. Such a phenomenon wherein the chiral nematic liquid crystal reflects circularly polarized light having a certain specific wavelength by the helical pitch and the average refractive index of the liquid crystal, is referred to as selective reflection.
The selective reflection wavelength (xcex) is substantially equal to a product (xcex=nxc2x7p) of the average refractive index (n) of the liquid crystal composition and the helical pitch (p) of the liquid crystal composition. Further, the helical pitch can be calculated by p=1/(cxc2x7HTP) from the amount (c) of the optically active compound added and the constant (HTT (=Helical Twisting Power)) of the optically active compound. Thus, the selective reflection color is determined by the type and the added amount of the optically active compound.
The liquid crystal alignment showing the selective reflection includes a case wherein the helical axes are substantially completely perpendicular to the substrate surfaces (which will be referred to as perfect planar state), and a case wherein the average of helical axes of liquid crystal domains divided in a plurality, is substantially perpendicular to the substrate surfaces (which will be referred to as imperfect planar state or simply as planar state). The perfect planar liquid crystal alignment and the planar liquid crystal alignment show different reflection behaviors. With the perfect planar liquid crystal alignment, regular reflection is large against incident light, and it shows a very high reflection at a certain specific viewing angle. On the other hand, with the planar liquid crystal alignment, regular reflection is relatively small, and it shows a high reflection behavior within a relatively wide viewing angle. Further, as another liquid crystal alignment, the chiral nematic liquid crystal may take an alignment wherein helical axes of plural liquid crystal domains are aligned in random directions or in non-perpendicular directions to the substrates (which will be referred to as focalconic state). In the focalconic state, many chiral nematic liquid crystals show a weak scattering state, and will not reflect light having a certain specific wavelength as in the case of the selective reflection.
These liquid crystal states are stable even when no electric field is applied. Further, the selective reflection by the planar state or perfect planar state, is bright, since no polarizing plate is employed, and further, the viewing angle is also wide with the planar state. Thus, a liquid crystal optical element employing a chiral nematic liquid crystal and utilizing the selective reflection, is a liquid crystal optical element which is bright and has a wide viewing angle, whereby a power consumption is small, since the liquid crystal alignment can be maintained even in a state where no electric field is applied (i.e. capable of functioning in a memory mode).
The planar state and the focalconic state can be changed to each other by application of an electric field. The change from the focalconic state to the planar state takes place via liquid crystal alignment so-called homeotropic wherein the liquid crystal molecules will be substantially parallel with the direction of application of the electric field, and accordingly, the highest voltage will be required.
The present invention has an object to provide a liquid crystal optical element which has bright selective reflection, a high contrast and a low driving voltage and which can be used in a wide temperature range (which is particularly excellent in the quality maintenance at a low temperature), and a liquid crystal composition to be used for such a liquid crystal optical element.
The present invention provides a liquid crystal composition comprising an optically active compound of the following formula (1) and a nematic liquid crystal:
R1xe2x80x94X1xe2x80x94A1xe2x80x94C*HY1xe2x80x94(CH2)mxe2x80x94X2xe2x80x94A2xe2x80x94(X3xe2x80x94A3)nxe2x80x94(X4xe2x80x94A4)pxe2x80x94X5xe2x80x94R2xe2x80x83xe2x80x83(1)
wherein the symbols have the following meanings:
R1 is a C1-10 alkyl group, a C1-10 alkenyl group or a hydrogen atom;
R2 is a C1-10 alkyl group, a C1-10 alkenyl group, a hydrogen atom or a cyano group;
each of X1, X2 and X5 which are independent of one another, is xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94Oxe2x80x94 or a single bond;
each of X3 and X4 which are independent of each other, is xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Oxe2x80x94 or a single bond;
each of A1 to A4 which are independent of one another, is an unsubstituted trans-1,4-cyclohexylene group, a 1,4-phenylene group which may have at least one hydrogen atom substituted by a halogen atom, a pyrimidin-2,5-diyl group or a 1,3-dioxan-2,5-diyl group;
C* is an asymmetric carbon;
Y1 is a C1-2 alkyl group which may have at least one hydrogen atom substituted by a halogen atom, or a halogen atom;
m is an integer of from 0 to 5; and
each of n and p which are independent of each other, is 0 or 1.
Further, in the above liquid crystal composition, the nematic liquid crystal preferably has a refractive index anisotropy (xcex94n) of at least 0.18, a nematic isotropic phase transition temperature (Tc) of at least 70xc2x0 C. and a dielectric anisotropy (xcex94∈) of at least 5. Further, it is preferred that the above liquid crystal composition selectively reflects visible light in the planar state.
Further, the present invention provides a liquid crystal optical element having the above liquid crystal composition interposed between a pair of electrode-attached substrates.
In the above liquid crystal optical element, it is preferred that row electrodes and column electrodes are provided on one substrate and the other substrate, respectively.
Further, in the above liquid crystal optical element, it is preferred that the distance (the cell gap) of the pair of opposing electrodes is from 2 to 20 xcexcm.